Noise compensation and/or noise reduction in acoustic signals is an important issue, for example, in the field of speech signal processing. The quality of an audio signal, e.g. of a speech signal, is often impaired by various interferences stemming from different noise sources. Hands-free telephony systems or speech recognition systems, for instance, may be used in a noisy environment such as in a vehicular cabin. In this case, the voice signal may be interfered by background noise such as noise of the engine or noise of the rolling tires. Noise compensation methods may be used to compensate for the background noise thereby improving the signal quality and reducing misrecognitions.
Common methods for noise compensation and/or noise reduction usually involve multi-channel systems. For example, two-channel systems are used, wherein a first channel comprises a disturbed audio signal and a second channel comprises a noise reference signal.
FIG. 6 shows an example of such a system. Two microphones 605 are configured to detect a wanted signal of a wanted sound source, for example, a speech signal. A first microphone signal is output by a first microphone on a first signal path and a second microphone signal is output by a second microphone on a second signal path. The first and the second microphone signals comprise a noise components 603 and 604, respectively, originating from one or more noise sources and a wanted signal component originating from the wanted sound source. The transfer between the wanted signal and the first and the second microphone signals may be modeled by a first and a second transfer function 601 and 602, respectively. The second microphone signal is filtered by an interference canceller 609, which comprises an adaptive filter and determines an estimate for the noise component in the first microphone signal based on the second microphone signal. The output of the interference canceller 609 is subtracted from the first microphone signal by a subtractor 610, thereby obtaining an output signal with reduced noise. The quality of the output signal depends on the wanted signal component in the second microphone signal.
In an ideal case, the second microphone signal and hence the output of the interference canceller 609 do not comprise a wanted signal component. The quality of noise compensation in the output signal with reduced noise, however, also depends on the correlation between the noise components 603 and 604. A low correlation implies that the estimate of the interference canceller 609 is a bad estimate for the noise component of the first microphone signal and that therefore the quality of the output signal with reduced noise is low. To achieve a higher correlation, and hence a better estimate for the noise reference signal, the two microphones 605 should have a small relative distance from each other. As a consequence, however, the second microphone signal will also comprise a significant wanted signal component.
In order to solve this problem, current multi-channel systems primarily make use of a so-called “blocking matrix” in order to block a wanted signal component in the second signal path.
FIG. 7 shows such a system comprising two microphones 705, an interference canceller 709 and a first subtractor 710 configured to subtract the estimate of the noise component from a first microphone signal. The first microphone signal from a first signal path may be used as input for an adaptive filter 715. The output of the adaptive filter 715 may be combined with a second microphone signal using a second subtractor 716, thereby obtaining a noise reference signal on a second signal path. This noise reference signal may be used as an input for the interference canceller 709 and the output of the interference canceller 709 may be subtracted from the first microphone signal using subtractor 710 to obtain an output signal with reduced noise. The first and the second microphone signal may comprise a noise component 703 and 704, respectively.
A first transfer function 701 modeling the transfer between a wanted signal and the first microphone signal on the first signal path may be denoted by G1(ejΩ) and a second transfer function 702 modeling the transfer between the wanted signal and the second microphone signal on the second signal path may be denoted by G2(ejΩ). Here j denotes the imaginary unit and Ω denotes a frequency variable. In order to obtain a noise reference signal with little or no wanted signal component, a transfer function, H, of the adaptive filter 715 may readH(ejΩ)=G2(ejΩ)G1−1(ejΩ)
In other words, the above-described transfer function of the adaptive filter 715 comprises an inverse of the first transfer function. This can yield an impaired noise reference signal if the value of the first transfer function approaches zero. This effect can result from room acoustics. If there is a strong reflecting boundary near a microphone, there are essentially two paths to the microphone: a direct path and a reflected path. Since the lengths of the two paths differ, the respective sound arrives at the microphone with a difference in phase. Depending on the frequency of the sound, the phase difference may either lead to constructive or destructive interference. Destructive interference can cause the signal to be destroyed at a particular frequency. In the art, this is referred to as a comb-filter because the destructive interference occurs periodically along the frequency axis. As a consequence the magnitude of the transfer function looks like a comb. There may be multiple such frequencies where the room transfer-function shows zeros depending on the delay between the direct path and the reflected component. It should be recognized that this discussion has been simplified, as there will be more that two paths.
Other known methods for determining a noise reference signals may similarly yield an impaired noise reference signal. The quality of noise compensation and/or noise reduction, however, depends to a large extent on the quality of the noise reference signal. Therefore, there is the need to provide a method for determining a more accurate noise reference signal for noise compensation and/or noise reduction.